Had I presumed to hope.
Counterfactual grammar
Past perfect subjunctive—'Had I presumed' means she didn't. The whole poem builds on something that never happened.
Giants gone away
Biblical echo of Numbers 13:33—the giants in Canaan. Loss of something huge becomes proof it was real.
Diligent Despair
Oxymoron—despair is usually passive collapse, but hers works hard. She's making not-hoping into an active discipline.
Celestial lists
Tournament language—'lists' are jousting grounds. She's fighting for heaven with earthly weakness.
Justified by Death
Legal/theological term—'justified' means declared righteous. Protestant doctrine: you're saved by grace, proven by death.